Your interest will evaporate as Frances McDormand and Matt Damon try to persuade a small, Middle America farming town to let their gas company drill, baby, drill in “Promised Land.”

In “Promised Land,” Hal Holbrook speaks up at a town meeting to say others don’t know jack about the plans to frack on their land.

I have to admit I was wrong about Matt Damon and John Krasinski’s anti-fracking movie “Promised Land.” When plot details first leaked out, I thought it couldn’t be that ridiculous. My bad!

Damon and Krasinski co-wrote the script, from a story by Dave Eggers, and play rivals on opposite sides of the fracking debate in this groaner of an agenda movie directed by Gus Van Sant, who guided Damon to stardom in “Good Will Hunting.” Example of the movie’s heavy-handed symbolism: Damon’s character, a steward to the mighty, is named “Butler.” Krasinski’s? “Noble.”

After first stopping in a rural store where they buy yokelwear to try to blend in, two weaselly gas-company pitchpeople (Damon and Frances McDormand) try to get a small farming town to sell drilling rights to the evil energy company Global.

They run into resistance from a plain-spoken town elder (Hal Holbrook) who steps out of a Norman Rockwell painting to tell a town meeting how fracking — breaking up rocks deep under the surface to release natural gas deposits — will poison the drinking water. Then Krasinski turns up as an environmentalist who, having grown up on a farm where all the cows were killed by frackers, tries to steer the townsfolk against the gas company.

Damon’s wisest move was casting himself. His smile has always seemed forced and his supposed all-American boyish look carries a smarmy undertone that is sheer Amway salesman. There’s a reason why his hits tend to feature him as an antisocial, violent loner (“Good Will Hunting”), a ruthless killing machine alienated even from his own personality (the Bourne movies) or a devious homicidal rat (“The Departed”).

But despite a promising start — a scene in which Butler quietly outfoxes a local pol with a “consulting fee” is a vivid illustration of power dynamics — the movie gets a case of the sillies. When Noble turns up, Butler, the smooth operator, turns into an unhinged alcoholic who gets into bar fights with the people he’s trying to win over and offers the environmentalist an envelope stuffed with cash, without even getting him to sign a contract. Noble pockets the money and goes right on with the rabble-rousing. (That’s why major corporations don’t do business like the Godfather — they don’t have any hit men on the payroll to enforce informal contracts.)

Damon wants to put you in Very Serious mode and have you think of “issue movies” like “The China Syndrome,” but all I could do was laugh and think of “Lolita.” Butler is like Humbert Humbert, the slimy degenerate trying to get away with shady doings in retro motels, which makes Krasinski the impish Clare Quilty-like foil who unnerves him by capering around the perimeter and stealing his girl (Rosemarie DeWitt). For Damon and Van Sant, Middle America is as strange as it was to Vladimir Nabokov.

As the movie goes on, Damon gives himself dialogue to twirl one’s moustache by (“We’re a $9 billion company: Do you know what we’re capable of?”). The lefty paranoia of the absurd third act, complete with Damon’s doomed attempt to deliver an Al Pacino moment, will remove any suspicions you may have that what you’re witnessing bears relation to reality.

Much has been made of the fact that “Promised Land” was partly funded by the enemies of our domestic gas industry — the foreign oil nabobs in the United Arab Emirates. But the film gets so cheesy that I suspect it was also secretly funded by Velveeta.